[fpc-devel] Definition of the time-fration in a TDateTime before 30/12/1899

[Joost van der Sluis]

On Sun, 2006-05-21 at 16:40 +0200, Michael Van Canneyt wrote:
> On Sun, 21 May 2006, Joost van der Sluis wrote:
> > I have some questions about the TDateTime format.
> >
> > I always thought that you could do this:
> >   TD : TDateTime
> >
> > TD := EncodeDate()+encodeTime();
> >
> > This would mean that you have the following: (24-hours notation)
> >
> >  0     = 12/30/1899 00:00    (as defined)
> >  1     = 12/31/1899 00:00
> >  1.25  = 12/31/1899 06:00
> > -1     = 12/29/1899 00:00
> > -2     = 12/28/1899 00:00
> > -1.25  = 12/28/1899 18:00    (-2+0.75)
> >
> > But now the Delphi help says:
> >
> > -1.25  = 12/29/1899 06:00
> 
> It's also what Delphi does.
> 
> >
> > And this is how it's also implemented in FPC. But for instance
> > PostgreSQL which also uses a format like TDateTime does it the other
> > way. Which looks more logical to me, since if you want to know if
> > DateTimeA is after DateTimeB you can do: DateTimeA>DateTimeB
> >
> > With the Delphi-help variant this won't work. Since -1>-1.25 but
> > 12/29/1899 00:00 > 12/29/1899 06:00 doesn't make sence to me.
> >
> > This implementation will mean that if you compare TDateTime's you first
> > have to check whether the date is before or after 12/30/1899 ?
> >
> > There are some bugs in DecodeDate, DecodeTime and friends for dates
> > before 12/30/1899 but to fix those, I need to know what to do with this
> > problem.
> 
> Keep delphi compatibility.
> 
> Also windows compatibility. TDateTime is a windows construct.
> (see variants)
> 
> They obviously judged that the idea of
>  TD := EncodeDate()+encodeTime();
> was more important than strict ordering.

That also doesn't work. If td < 0 you must do: 
  TD := EncodeDate()+(1-EncodeTime());

> The following remark in the Delphi help sheds more light on the issue:
> 
> "When working with negative TDateTime values, computations must handle
> time portion separately. The fractional part reflects the fraction of
> a 24-hour day without regard to the sign of the TDateTime value.
> For example,
>   6:00 am on 12/29/1899 is -1.25,
> Not
>   -1 + 0.25,
> which would be -0.75. There are no TDateTime values between -1 and 0."

That explains why -0.5 is the same as 0.5.
Where in the help can I find that?

> So they are aware of the issue.

Ok, I'll fix the datetime utils with this in my mind.

Joost.